The correlation between milk mastitis and electrical conductivity of the milk is well established. There exist many references in both the scientific and patent literature for determining the presence of mastitis from measured electrical conductivities of a cow's milk. Thus, for example, British Patent No. 1 438 281 describes a mastitis detector based on the electrical conductivity of a cow's milk and comprising an improved conductivity cell.
British Patent No. 1 494 831 is directed to a method and apparatus for the early detection of mastitis, for example, whilst in the sub-clinical stage enabling corrective action to be taken before the infection has any significant effect upon the quality or quantity of milk yield. The method disclosed in the above-mentioned patent is based on the finding that in practically all cases of mastitis the level of infection is different in at least two of the four teats of a cow for at least a period of time equal to that between successive milkings. Based on this fact, a comparison of the impedance of the milk drawn from each teat at each milking is used to detect the onset of mastitis.
It is further stated that the constant monitoring to detect mastitis may also serve to detect general infection of a cow by comparing the individually measured conductivities with a reference conductivity corresponding to a norm derived from a sample of pure milk from a healthy cow. It is thus suggested that the onset of mastitis in a cow may be determined with reference to the milk conductivities associated with a different, healthy cow.
Similarly, U.S. Pat. No. 4,325,028 analyzes the conductivities of milk drawn from quarter mammae by subtracting the minimum conductivity of the four mammae from the conductivities associated with the remaining three. The resulting differential conductivities are compared with predetermined reference values, mastitis being indicated when any one differential value is greater than an upper threshold. In a practical system, the differential conductivities are compared with a lower threshold of 5.times.10.sup.-4 S/cm the higher threshold being 15.times.10.sup.-4 S/cm, no support being given in the referenced patent for either of these figures.
Underlying the methods employed in the above-mentioned prior art references is an assumption that there is some normal value of electrical conductivity which may be used as a yardstick against which the milk conductivities associated with individual cows may be compared in order to assess the onset of mastitis. It has been found, however, that this basic assumption is too simplistic to provide meaningful data in anything but clear-cut cases of mastitis. Thus, for example, whilst the above-mentioned global conductivities of 5 and 15 m /cm can indeed be used as an indication that a measured sample of milk is mastitic, they are too broad to permit prediction of mastitis or to indicate the onset of mastitis in its incipient stages.
It is obviously desirable to predict the likely onset of mastitis, if possible, so that suitable remedial action can be taken and the cow restored to full health without incurring the inevitable loss of milk yield which accompanies mastitis. However, the prior art is concerned only with improved systems for measuring electrical conductivity or for methods which establish mastitis once it has already occurred, no method having yet been suggested for analyzing electrical conductivity data so as to predict the likely onset of mastitis.
It is, in fact, difficult to define criteria which predict the onset of mastitis for the following reasons:
1. the electrical conductivity of a cow's milk does not remain constant even throughout the same milking; PA1 2. the electrical conductivity associated with a cow's milk varies throughout the day so that, for example, the conductivity of the morning milking may well be different from that of the evening milking; PA1 3. the conductivity of the cow's milk depends on factors apart from the general health of the cow such as, for example, the kind of feed consumed by the cow, ambient conditions such as climate, and so on; PA1 4. the conductivity of a cow's milk increases towards the end of the cow's lactation; and PA1 5. the milk conductivity associated with a healthy cow lies within the range 9-15 m /cm. This range is sufficiently broad that significant fluctuations in the electrical conductivity of an individual cow's milk can occur within this range without the cow being classified as unhealthy. PA1 (a) measuring at least one electrical conductivity of at least one daily milking on each of a plurality of successive days and storing the respective electrical conductivities or computed functions thereof, PA1 (b) computing and storing an average value of the electrical conductivities or functions thereof stored in (a) for the preceding x days but excluding the previous y days where y&lt;x, PA1 (c) at least once each day computing any deviation between the electrical conductivity or computed function thereof determined in (a) and the average value computed and stored in (b), and PA1 (d) storing and/or displaying any deviation computed in (c).